That's fine. It's my choice whether to upgrade my OS or not. If I do so, and my app no longer works on the new OS, then I can purchase a new version. Why should I pay a monthly fee just in case I might upgrade my OS and just in case the app doesn't work there?

Also, it's far less likely than you're suggesting that I would need the exact major OS version I started with. The majority of apps survive an OS upgrade just fine. In my case, I use NixOS, so that isn't even an issue at all since every package can have all of its dependencies frozen for as long as you like, even with a new major OS version.

It’s worse than that, stop paying Apple $99/year and all your apps get pulled.